Tuesday, April 5, 2016

Nenana Ice Classic

The 100th Nenana Ice Classic closes to entries today, and I'm guessing those entries are heavily front-loaded towards an early breakup, given the warm weather of recent weeks and months in interior Alaska.  The ice is slightly thicker than it was last year at this time, but the visual appearance has been quite poor, judging from the webcam view in recent weeks.  It's actually looking a bit better today thanks to a little fresh snow on Saturday night.


I've mentioned before that April mean temperatures in Fairbanks are highly correlated with the breakup date at Nenana - see the chart below.  The warmest April in Fairbanks history was in 1940, and that year also produced the earliest breakup on record - narrowly ahead of 1998.  However, the early breakup of 1940 was quite an outlier for those early decades; 6 of the earliest 7 breakups have occurred since 1990.  In the past few decades, it's been more common to see breakup in April than in May, in contrast to earlier years.


Another way of looking at the relationship between breakup and temperature is to calculate the thawing degree days up until the date of breakup.  Thawing degree days (TDDs) are the accumulation of temperature excess above freezing; for example, a daily mean temperature of 42°F equates to 10 TDDs.  If we add up the daily TDD values for each year between March 1 and the date of breakup, we get the following chart:


The first thing to note is that there is quite wide variation in the amount of thermal energy needed to produce breakup, as measured by TDDs; since 1930, the Fairbanks TDDs have varied from 75 (in 2002) to 265 (in 1975).  It's slightly counter-intuitive to see that the latest breakups occur after a relatively small accumulation of TDDs; the reason for this (I believe) is that late breakups allow more time for the increasingly intense solar radiation of May to work on the ice, even though air temperatures are low.

In contrast, if breakup is to occur early, then melting has to be accomplished more by the warmth of the air than by direct solar heating, and so warmer temperatures are required.

We can see that recent decades have been characterized more by early breakups in association with large TDDs, in contrast to earlier years that tended to have smaller TDDs and later breakup.  The last two years, 2014 and 2015, were actually a little unusual in having early breakup AND relatively low TDDs - perhaps a consequence of the warm winters and poor ice conditions.

So far this spring Fairbanks is accumulating TDDs more rapidly than in the past 3 years, although it's not a record; in 1965, 45 TDDs were observed already by April 4 (but breakup didn't occur until May 7).  According to today's 7-day NWS forecast, Fairbanks will accumulate another 29 TDDs through April 11, putting the total at 59.  There doesn't seem to be any sign that the warm weather will abate in subsequent weeks, so the melting rate will probably accelerate in the middle of April, especially as snow cover will be gone by then.

Subjectively, then, it seems we are on pace for something pretty close to a record early breakup, but much will depend on just how warm it is after April 15.  I'd say the most likely window for breakup at Nenana is April 21-24; and that's probably close to what everyone else is thinking without any of the temperature analysis.  In other words: your guess is as good as mine.

7 comments:

  1. Today's GFS MOS forecast is suggesting Fairbanks will see 100 TDDs by April 13, or day 44 on the second chart. Record early breakup is looking a little more likely.

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  2. Thanks Richard. Always nice to read your report!

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  3. I flew the river yesterday. The Tanana River near Fairbanks is running water in the channel center. After about 10 miles the flow is under-ice but there's surface flow along both river banks from snow melt and tributary input all the way to Nenana. The Nenana River is ice covered but with discontinuous melting. About 25 miles west of Nenana it's still winter with no surface melting on the area lakes. Large rivers tributary to the Tanana show discontinuous surface melt. And today it's snowing over the middle Tanana Valley.

    Upstream of Nenana it's Spring; downstream not very much.

    Gary

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    Replies
    1. Thanks for the report, Gary. The east-west temperature gradient has been remarkable: Tanana has seen only 2 days above 40F so far; Fairbanks has had more days (4) above 50F.

      Interesting to see the snow today. Remarkably the computer forecast still shows highs of 53-57F for the next 7 days.

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    2. Warm SE flow through the Alaska Range passes is affecting a portion of the Tanana Valley's weather. When it gets no colder than the lower 30's at night it'll finally be Spring for all.

      Gary

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  4. April 11, 2016 Alaskan River Breakup Forecast. Earlier than usual?:

    http://aprfc.arh.noaa.gov/products/fcst.php?product=FGAK78PACR

    Gary

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